The assisted recovery of petroleum is commonly used in the petroleum industry to recover crude oil that remains in place in a formation after its natural or forced production or to recover heavy fuel that is too viscous to flow naturally or artificially using unsophisticated pumping means. Traditionally, steam was the driving force that was most commonly used to flush crude oil formations effectively, but production programs have increasingly implemented an injection of carbon dioxide (CO2) to produce petroleum. Carbon dioxide has proven to be a particularly advantageous gas, to the extent that it can be used by the producers for the assisted recovery of petroleum.
Injection of CO2 specifically for the purpose of increasing hydrocarbon production, is known as Enhanced Oil Recovery (EOR) or “assisted recovery”, see U.S. Pat. Nos. 4,333,529; 3,065,790; and 3,150,716. Carbon dioxide is introduced into an injection well proves effective for reducing the viscosity of the petroleum in place and for increasing its mobility, which facilitates its recovery.
Desirably, the recovered CO2 from a production well can be reinjected via an injection well to reduce the viscosity of the petroleum in place in a formation. However, a non-negligible portion of CO2 is sequestered in the formation and irretrievably lost, a portion of the injected carbon dioxide is dissolved in the crude oil solution and can therefore be recovered with the petroleum during its production to be recycled later, and the remaining CO2 stream that is recovered from a production well contains hydrocarbons.
The recovered CO2 stream is a gaseous and liquid stream that is under pressure and typically is separated in gas-liquid separators, to obtain a liquid phase of propane and heavier hydrocarbons (C3+ hydrocarbons), and a gaseous phase containing primarily CO2 and a substantial amount of impurities of methane (C1) with a little less ethane (C2). These impurities can represent 5 to 12 mol percent of the separated gaseous phase. In some cases where acidic crude oils are produced, the associated gas can also contain the hydrogen sulfide (H2S) that is found in part as an impurity (several percent, for example). The processes of U.S. Pat. No. 4,762,543 and US Patent Application No.: 2002/0036086 are directed to a process for removal of such components from the produced CO2 prior to its reinjection.
Another possibility would be to reinject the impure CO2 stream that contains methane and ethane into the formation, such action would deleteriously affect the saturation pressure of the formation. Other possibilities would be to mix impure CO2 stream with pure CO2 obtained from an outside source, or with other hydrocarbons that are heavier than the impurities in such a way as to dilute it and to counterbalance the volatility of the methane, but these technical solutions are very costly.
Cryogenic distillation could also be used to extract the methane and the ethane from CO2 and then the separated CO2 could be recycled into the formation. This separation, however, also proves to be very expensive.
Solvents exist that can absorb CO2, preferably with hydrocarbons, but these solvents would very easily absorb the hydrogen sulfide that is present, which would produce CO2 that is polluted by H2S.
U.S. Pat. No. 4,344,486 teaches hydrocarbons that contaminate carbon dioxide can be combusted by an oxygen-enriched gas or by essentially pure oxygen.
There exists a need for an effective and inexpensive separation technique that is capable of removing the full range of hydrocarbons without adding new contaminates.